1. Technical Field
The present invention relates to an optical imaging system for pickup, more particularly to the optical imaging system for pickup with six lens elements to reduce total track length and improve image quality for applying to electronic devices.
2. Description of Related Art
In compact electronic devices such as digital still cameras, or mobile phone cameras, an optical imaging system for pickup is generally installed for capturing images of an object, and the optical imaging system for pickup tends to be developed with a compact design and a low cost, while meeting the user requirements for good aberration correction ability, high resolution, and high image quality.
In general, a conventional optical imaging system for pickup of a compact electronic device comes with different designs, including the two-lens, three-lens, four-lens, five-lens and six-or-more lens designs. However, if the imaging quality is taken into consideration, the optical imaging system for pickup with the four-lens, five-lens or six-lens designs has advantages on image aberration and modulation transfer function (MTF) performance, wherein the six-lens design having a higher resolution than the four-lens, or five-lens designs is suitable for electronic devices requiring high quality and high pixels.
In various compact designs of the six-lens optical imaging system for pickup having a fixed focal length, prior arts adopt different combinations of positive and negative refractive powers as disclosed in U.S. Pat. No. 5,682,269 and U.S. Pat. No. 5,513,046 adopting for a group of stacked lenses. For example, the first lens element with negative refractive power and the second lens element with positive refractive power are used for shortening the total length of an optical system as disclosed in U.S. Pat. No. 7,564,634.
In products such as compact digital cameras, web cameras, and mobile phone cameras, the optical imaging system for pickup requires a compact design, a short focal length, and a good aberration correction. Wherein, the six-lens optical imaging system for pickup having a fixed focal length generally adopts the sixth lens element with positive refractive power to allow the increase of the view angle so as to reduce the total length of the optical imaging system. As disclosed in U.S. Pat. No. 7,701,649, U.S. Pat. No. 4,389,099, and U.S. Pat. No. 4,550,987, adopting the optical imaging systems tend to have a good aberration correction, but the total length of the optical imaging system for pickup still fails to satisfy the specifications for compact electronic devices. In addition, a combination of the fifth lens element with negative refractive power and the sixth lens element with positive refractive power is adopted, and the refractive power and the rear focal length of the optical imaging system for pickup are adjusted to avoid aberrations caused by excessive positive refractive power as disclosed in U.S. Pat. No. 3,997,248. These conventional designs must increase the rear focal length of the optical imaging system for pickup, so that the total length of the optical imaging system cannot be reduced.
Therefore, the present invention provides a more practical design to shorten the optical imaging lens assembly adopts a combination of refractive powers of six lens elements, convex and concave optical surfaces. Wherein the fifth lens element and the sixth lens element have positive refractive power and negative refractive power respectively, and this complementary combination with a telecentric effect is favorable for reducing the rear focal length and the total length of the optical imaging system for pickup effectively as well as further improving the image quality and applying the optical imaging lens assembly to compact electronic products.